760 GENERATION AND DEVELOPMENT. 



and from the placenta, contains the remnants of other 

 structures which in the early stages of the development of 

 the embryo were, as already related, of great comparative 

 importance. Thus, in early foetal life, it is composed of 

 the following parts : (i). Externally, a layer of the 

 amnion, reflected over it from the umbilicus (fig. 219). 

 (2). The umbilical vesicle (10, fig. 219), with its duct and 

 appertaining omphalo-mesenteric blood-vessels. (3). The 

 remains of the allantois, and continuous with it the urachus. 

 (4). The umbilical vessels, which, as just remarked, ulti- 

 mately form the greater part of the cord. 



DEVELOPMENT OF ORGANS. 



It remains now to consider in succession the development 

 of the several organs and systems of organs in the further 

 progress of the embryo. 



Development of the Vertebral Column and Cranium. 



The primitive part of the vertebral column in all the 

 Yertebrata is the gelatinous chorda dorsalis, which con- 

 sists entirely of cells. This cord tapers to a point at 

 the cranial and caudal extremities of the animal. In the 

 progress of its development, it is found to become enclosed 

 in a membranous sheath, which at length acquires a fibrous 

 structure, composed of transverse annular fibres. The 

 chorda dorsalis is to be regarded as the azygos axis of 

 the spinal column, and, in particular, of the future bodies 

 of the vertebrae, although it never itself passes into the 

 cartilaginous or osseous state, but remains enclosed as in a 

 case within the persistent parts of the vertebral column 

 which are developed around it. It is permanent, however, 

 only in a few animals : in the majority it disappears at an 

 early period. 



The cartilaginous or osseous vertebrae are always first 

 developed in pairs of lateral elements at the sides of the 



